1. Field of the Invention
The present invention relates to a method of detecting a knock that occurs in an internal combustion engine based on the characteristics of an ionic current.
2. Description of the Related Art
A typical method of detecting a knock occurring in an internal combustion engine utilizes a vibration-type knock sensor for detecting such a knock. Signals from this knock sensor are judged in a predetermined section to detect knocks. More specifically, signals in a predetermined frequency band are taken out of the signals from the knock sensor and then processed for the detection of knocks.
Another approach has been attempted recently to detect a knock by passing an ionic current in a combustion chamber immediately after ignition and detecting a knock component overlapping the ionic current. One known such approach is such as to separate a knock component overlapping the ionic current and determine whether a knock occurs or not based on the knock component thus separated, as disclosed in, for example, Japanese Patent Laid-Open Gazette No. H6-159129. On the other hand, the art described in Japanese Patent Laid-Open Gazette No. H11-2174 establishes a reference level for judgment of the occurrence of a knock based on the mean value of an ionic current up to that time and compares a knock component with the reference level thereby to detect the occurrence of a knock.
In the art of detecting a knock by using an ionic current as above, the waveform of the ionic current to be detected is found to vary depending upon the electrical characteristics of a processing circuit and other conditions upon detection. An ionic current waveform attains its peak or maximum with substantially the same timing with the peak of combustion at which the combustion pressure reaches its maximum. In general, a knock component overlaps the ionic current at a point past the peak of the ionic current waveform.
If, for example, the peak value of an ionic current is substantially doubled due to variations in driving conditions or detecting conditions, a knock component overlapping the ionic current also is substantially doubled. Accordingly, if the knock-judging level is established based on such an inconstant ionic current, the knock-judging level thus established is also inconstant and, hence, learning becomes necessary every time the ionic current is detected. Since the peak of the ionic current varies substantially simultaneously with the peak of the combustion pressure, the learning of the knock-judging level cannot absorb fluctuations of the knock-judging level unless such learning is conducted in each driving state of the internal combustion engine. Such learning, however, is required to vary to accommodate different learning regions established according to driving regions and is conducted for all the learning regions and, hence, control therefor becomes complicated. Further, the latter prior art technique has a limitation in judging accuracy because the judgment of the occurrence of a knock in an occasion of combustion is effected with reference to a reference level established based on the ionic current in occasions of combustion preceding the occasion of combustion for detection.
The present invention intends to overcome such inconveniences.
According to the present invention, there is provided a method of detecting an occurrence of a knock in an internal combustion engine by detecting an ionic current flowing in a cylinder of the engine from initiation of combustion therein and judging a knock component overlapping the ionic current based on a judging level, the method comprising the steps of: detecting a peak value of the ionic current; and relatively correcting at least one of the judging level and the knock component based on the peak value thus detected to make more difficult a judgment of whether the knock occurs or not as the detected peak value increases.
The method makes more difficult the judgment of whether a knock occurs or not as the detected peak value increases by relatively correcting at least one of the knock component and the judging level. Accordingly, even when the knock component is apparently enlarged to a level higher than its real level with increasing peak value of the ionic current, such a knock component apparently higher than its real level can be excluded from the detection of a knock. Thus, the method offers an improved knock detection accuracy. For instance, the method will not make a detection error such that a knock component that is really too small to judge as a knock but appears to be enlarged to a value higher than its real level due to a peak value of the ionic current increased by fluctuations in driving state is erroneously detected as an indication of the occurrence of a knock. Thus, the method is capable of detecting only knock components indicative of real knocks thereby accurately detecting occurrences of knocks. Further, the method is capable of detecting a knock occurring in an occasion of combustion based on the value of a peak appearing in the same occasion thereby detecting occurrences of knocks without any detection error even in a transient period of combustion in which the driving state is inconstant.
In the above method, the correction of at least one of the judging level and the knock component may be made in at least one of a direction such as to decrease the knock component as the peak current increases and a direction such as to raise the judging level as the peak current increases. Since the correction is made such that the knock component is relatively decreased as the peak value of the ionic current increases, the method having this feature can assuredly prevents a detection error even if an apparent knock appears to have a magnitude equal to the magnitude of a real knock and hence enjoys an improved detection accuracy.
Preferably, the judging level is determined based on a reference level for judging the knock component, the reference level being calculated from a peak value of the ionic current in a normal combustion free of the knock. The method having this feature is capable of correcting the judging level to a proper level by correcting the calculated reference level obtained in the normal combustion free of a knock with an actually measured value. Advantageously, the method has a feature such that the judging level is determined as a value obtained by multiplying the reference level by a factor of a positive number N and compared with the knock component or the judging level is determined as being equal to the reference level and compared with a value obtained by dividing the knock component by a factor of a positive number M. With such a feature, the judging level can be properly established in a manner to accommodate variability of the reference level. When the factor by which the reference level is multiplied or the knock component is divided is variable according to driving states, for example, according to the number of revolutions of the engine or the load imposed on the engine, a further improvement in detection accuracy will result.
In order to minimize the influence of noise produced in an actual driving state of the internal combustion engine, it is preferred that the method further comprise the step of detecting a noise component overlapping the ionic current in a driving state free of a noise during a knock component detecting period, wherein the correction of one of the judging level and the knock component is made based on the noise component thus detected. This feature makes it possible to judge a knock with exclusion of different noises, particularly white noises or analogous noises, which result from variability in the manufacture of an electric circuit adapted to process the ionic current or from different driving states, thereby bringing an improved judgment accuracy.
The foregoing and other objects, features and attendant advantages of the present invention will become apparent from the reading of the following detailed description with reference to the attached drawings.